1. Field of the Invention
This invention relates to sewing machines and, more particularly, to a bobbin assembly on a sewing machine which causes improperly routed thread to sever before damage to, or impairment of the operation of, the sewing machine caused by the improperly routed thread can occur.
2. Background Art
Bobbin assemblies are used in a wide range of sewing machines. These bobbin assemblies typically have a receptacle within which a center post is formed to mount a thread retaining bobbin. In some systems, the bobbin axis is vertical, while in others the bobbin axis is horizontal.
The bobbin assembly typically has an element which is driven in rotation, continuously or in an oscillating manner, to define a loop in thread presented by a needle, through which stitching between the needle and bobbin threads is carried out. These bobbin assemblies commonly include a bobbin case/basket which fits in the receptacle or a cap which captively retains a bobbin in the receptacle. A pivotable latch element may be provided on the center post to maintain the cap in place and in turn the bobbin in its operative position in the receptacle.
Operator error accounts for a considerable amount of damage to sewing machines, which often results in significant down time. One operator induced problem results from either the improper securement of a bobbin case in its receptacle or failure to place the bobbin case in its receptacle prior to the operation of the sewing equipment. In equipment with a horizontal bobbin axis, the failure to place and lock the bobbin case/basket in its proper operative position, or to latch a bobbin in place, may result in the bobbin case escaping from its receptacle during operation of the sewing equipment. In a vertical axis system, operators at times may fail to install the bobbin cap or bobbin case in the basket or may leave the pivotable latch member in an unlatched state.
In all the above situations, and others which also occur, as the sewing machine is operated, thread in a loop formed by the bobbin assembly operating in either the continuous or oscillating manners, as it constricts, tends to route improperly and wrap around the center post. The thread take-up mechanism can cause a significant tension to be produced on the thread, particularly in heavy duty equipment which is designed to sew thick materials, such as heavy cloth and leather.
Many modern bobbin assemblies include non-metallic elements, such as a bobbin basket with an integrally formed center post. A constricted thread loop that has wrapped against the center post on these bobbin baskets may cause the center post to snap off the remainder of the basket. Since the initiated sewing operation may continue to run automatically until a particular cycle is completed or the overall system jams, the potential to inflict substantial damage on the center post, and elsewhere, exists. As an example, a 138 nylon thread may withstand in excess of 21.5 pounds before it breaks. This thread, aside from bending or breaking off the center post, may inflict damage to undertrimmer knives, hook raceways, thread retainer plates, and the hook elements before eventually causing a jam that stops the machine. Further, the stronger the thread the greater the potential is for damage.
While bobbin assemblies formed with metal center posts tend to withstand this condition better than those made from non-metallic materials, even the metal posts may become bent or broken in heavy duty applications. The metal latch element on the center post is particularly prone to being snagged and damaged by improperly routed thread.
Heretofore, nothing known to the inventor has been devised to address the above problem. It is known in the prior art to equip a bearing rib which guides rotation between the bobbin basket and a hook assembly, with a cutting element which severs thread before a substantial thread buildup occurs between the bobbin basket and the hook assembly. However, this structure does not cause severance of thread which is properly formed in a loop and which constricts to embrace the center post on the bobbin assembly in the absence of proper placement of bobbin cases or covers or proper situation of a movable latch on the center post. As a result, those in the sewing industry have generally contended with this problem by effecting the necessary repairs after the condition occurs, absorbing the losses attributable to this maintenance and down time.